It has been recently found that random copolymers of ethylene and specific bulky cycloolefins are excellent in transparency and have balanced heat resistance, heat ageing property, chemical resistance, solvent resistance, dielectric characteristics and mechanical properties, and such random copolymers was proposed in U.S. Pat. No. 4,614,778 and Japanese patent Laid-open Publication No. 61-98,780 (1984). While the random copolymers proposed have excellent properties as described above, they are likely to contain an amount of unreacted cycloolefin monomer, which impair the quality of the products. Japanese Patent Laid-open Publication No. 62-215,611 discloses a process for removing the unreacted cycloolefin monomer from the ethylene-cycloolefin random copolymers.
By the process disclosed in Japanese Patent Laid-open Publication No. 62-215,611, there can be obtained cycloolefin random copolymers of high quality which are suitable for use in optical materials, in particular optical memory discs. This process is, however, complicated, and is hardly productive of a purified product of a uniform quality unless the starting copolymer have a molecular weight within a certain limited range.
Because of their excellent properties ethylene-cycloolefin random copolymers find their application in various other fields, and thus, desired in the art is a process for removing unreacted cycloolefin monomer from ethylene-cycloolefin random copolymers having a molecular weight within a wide range.
Further, the inventors have found some problems that upon continuous copolymerization of ethylene with a bulky cycloolefin in a polymerization vessel equipped with a stirrer, although depending upon the reaction conditions, undesirable copolymers which have unduly high content of ethylene and are insoluble in a hydrocarbon solvent used in the polymerization reaction (referred to hereinafter as solvent-insoluble copolymers) are liable to be formed on the wall of the polymerization vessel in the vicinity of the gas-liquid interface in the vessel, that the solvent-insoluble copolymers formed on the wall of the vessel in the vicinity of the gas-liquid interface invite changes in conditions of the gas-liquid interface with time and, when formed in large amounts, result in decrease of the effective area of the gas-liquid interface, that on that account the copolymerization of ethylene and the cycloolefin does not proceed sufficiently, and that the solvent-insoluble copolymers formed on the wall of the vessel fall off in the liquid phase in the vessel, withdrawn through a pick up line together with the desired copolymer produced, and trapped by a filtering device installed in the pick up line to clog the filtering device and occasionally the pick up line in itself, thereby hindering a continuous and stable running of a series of apparatus for the production of the ethylene-cycloolefin random copolymers including the filtering device.